In chapter 4 of "Lord of the Flies", when Ralph is mad at Jack for letting the fire go out, Simon says that he is afraid. Why is he afraid?

Simon does not say that he is afraid, the narration says that he "...looked now, from Ralph to Jack, ..., and what he say made him afraid."  Simon is the first one in the story to realize that the biggest fear the boys have is of their inner savagery.  He realizes it is no outside beast who will bring them down; it is themselves that will bring them down.  He sees the anger on Ralph's face and the brutality on Jack's painted face.  He seems to sense that these two boys represent the opposing forces on the island and that the brutal, savage force (Jack) will win.  Jack has painted his face for the first time, giving him a mask to hide behind as much as a mask to hide him from the pig. Simon has a good sense of right and justice, which is why he gives his portion of meat to Piggy at the end of the chapter.  This sense of right tells him that Ralph is right regarding the need for fire, but it also tells him that the boys are beginning to fall apart; that the savage force is beginning to take over.  He especially sees that in Jack, though he also understands the need for food that Jack provides.